Dehydration of acetic acid by azeotropic distillation



Dec. 10, 1946. H. M. GUINOT 2,412,215

DEHYDRATION OF ACETIC ACID BY AZEOTROPIC ,DISTILLATION Filed Jan. 8,1941 ETHYL AC TATE enema Dec. m, 1946 UNITED STATES, PATENT OFFlCEDEHYDBATION OF ACETI C ACID BY AZEOTROPIG DIS'IIILATION Henri MartinGninot, Niort, France; vested in the Custodia Alien Property ApplicationJanuary 8, 1941, Serial No. 373,689

In France January 13, 1940 (C1. MHZ) on the one hand, an acetic ester,such for example as ethyl acetate, and, on the other hand, an

auxiliary carrying liquid insoluble in water and forming therewith aheterogeneous azeotropic mixture having a boiling point lower that ofthe azeotrope ester-water. In this way there are constituted in thenormally running column two distinct zones situated above the feed inletfor the products to be dehydrated. In one of these zones operates theester which has for its object to prevent acid and anhydride from risingto the top of the column, while in the other zone operates the auxiliarycarrying liquid which takes hold of the water brought in by the esterand leads it away into a decanter in which said water owing to its beinginsoluble in ether is separated without difliculty, even in the presenceof soluble volatile impurities.

The applicant has found that the said process is applicable to thosesolutions of acetic acid and, possibly, of anhydride, which includestrong proportions of an acetic ester, such as are obtained for exampleby extraction of aqueous acetic solutions or by oxidation ofacetaldehyde in the presence of ethyl acetate. In such events, the sameester as thatcontained in the feed mixture is employed as a constituentof the carrying liquid. 1

However, in this case it is indispensable to make provisions, in thecourse of distillation, for discarding the ester continuously brought inby the starting material. To bring about such elimination, one shouldnot venture to think of extracting the excess ester in the usualdecanter connected with the top of the column, as is commonly practicedin all the processes of azeotropic dehydration employing an estersolely, for there would thus be determined through the column, condenserand decanter a continuous flux of ester which would prevent theformation of the two operative zones featuring the invention and therewould then be lost the benefit of the preponderant presence, inthedecanter, of the insoluble auxiliary carrying liquid.

According to the invention, the difliculty i resolved by withdrawing theexcess ester, preferably in liquid state, from the median zone in whichthe ester operates alone, that is to say, at a point where the liquidwhich boils on the column plates contains no auxiliary carrying liquid.It is advantageous to effect such withdrawal of ester on a plate ofincreased capacity specially provided in the zone and constituting areserve of liquid.

2 The product thus withdrawn is hydrated ester. It may be eitherre-employed directly as a solvent when it is dealt with the extractionof acetic according to known processes.

acid from its diluted solutions, or preliminarily dehydrated bydistillation or any other means when the treated liquid results from theoxidation of acetaldehyde in the presence of ethyl acetate The followingexamples, together with the accompanying drawing are given for thepurpose of illustration but not of limitation, in order to show how theinvention may be readily carried into practical effect.

Example I Into a column A provided with a condenser B and decanter C,and charged once for all, in accordance with the aforesaid U. S. A.patent application, with a suitable quantity of ethyl acetate andas'elected volatile oil boiling between 84 and 85 0., there isintroduced through the pipe I a mixture constituted by:

Percent Actetate of ethyl 82 Acetic acid 10 Water.. 8

from the median portion of the zone a a quantity of liquid regulated bymeans of the valve 6' and corresponding to the quantity of ethyl acetateintroduced into the column A by the mixture supply. The liquid withdrawnat 6 and cooled at H is extracted from the apparatus through the pipe 9and valve 9' and, possibly after decantation, is then immediatelyreintroduced into the extraction battery.

Anhydrous acetic acid is withdrawn at 5 from the foot of column A.

Example II There is introduced into the column A through the pipe I amixture constituted by:

Per cent Ethyl acetate 64.5 Water 3.9 Acetic acid 10.6 Acetic anhydride21.0

The column A being normally operated, there is withdrawn through thepipe 6 and valve 6' a.

quantity of ethyl acetate corresponding to that brought in by the feed.This liquid, which besides ethyl acetate includes a small quantity ofwater, is cooled at H and then introduced into the decanter D, whereinit is separated into two layers which consists of a heavier layer,mainly water and a lighter layer, an azeotropic layer. The watery layerwould pass to E where the water would pass off at 'l' and the lighterfraction would pass to G. The lighter layer from D would pass to Fwhence ethyl acetate contained therein would pass off at 8 and thelighter ingredients would join those from E passing into G. Thecondensed ingredients in G would either return to D to go through theoperation again or return to E to go through the cycle from that pointagain.

The temperature of the coils at the bottom of both E and F would beadjusted so that the vapors passing out of the towers E and F wouldcondense in G breaking down some of the azeotropic mixtures entering thetowers so that as they trickled downwardly to D they would stratify uponreaching D.

Nor will the spirit of the invention be departed from if the withdrawnester is dehydrated by means other than the distillation. For example,such ester may be treated directly in a small washing tower containingany dehydrating agent.

Finally, without departing from the scope of the invention, a specialplate may be provided in the zone a to permit of the liquid to bedecanted in the hot way within the colunm itself, according to thearrangement described in the French Patent No. 832,892. In this case,there will be withdrawn through the pipe 6 the upper layer obtained byhot decantation, that is to say, a product highly enriched in esterwhich a mere boiling in one smaller column will suflice to thoroughlydehydrate.

It may also be found advantageous to extract, besides the upper layerdecanted in the hot way, a certain proportion of aqueous layer whichafter distillation to recover the ester dissolved therein is discarded,thereby relieving the main column from supplemental burden of operation.

I claim:

l. A continuous process for producing an anhydrous acetic acid from asolution comprising a major portion of an acetic ester and aqueousacetic acid which comprises introducing the solution into the middlepart of a distilling zone containing predetermined proportions ofv thesame acetic ester and a water-insoluble water entraining oil adapted toform with water an azeotropic mixture of lower boiling point than theazeotropic mixture of said ester and water, heating the d stilling zoneat an end thereof whereby a temperature gradient is set up in said zoneforming therein a section of operation for the acetic ester and asection more remote from the heated end for said water-insoluble waterentraining oil; withdrawing the vapor from the end remote from theheated end, condensing the vapors, separating the non-aqueousconstituent from the condensed vapors and returning the same to theentraining liquid section; withdrawing liquid containing a predominantpercentage of said acetic ester from the acetic estersectionof thedistilling zone substantially at the same rate as the acetic ester isfed to the-distilling zone in the form of a component of said solution,and withdrawing 2. The process of claim 1, further comprising coolingthe liquid withdrawn from the ester operation section of the firstdistilling zone, decanting said liquid into two layers; and separatelydistilling said layers to recover anhydrous acetic ester. I

3; The process of claim 1, further comprising dehydrating the liquidwithdrawn from the ester operation section of the first distill ng zone,to recover anhydrous acetic ester.

4. A continuous process for producing an anhydrous acetic liquid from asolution comprising a major portion of an acetic ester and aqueousacetic acid which process comprises introducing the solution into themiddle part of a distilling zone containing predetermined proportions ofthe same acetic ester and a water insoluble water entraining oil adaptedto form with water an azeotropic mixture of lower boiling point than theazeotropic mixture of said ester and water, said oil being insoluble inwater; heating the distilling zone at an end thereof to set up in thesame a temperature gradient from said end to the opposite end therebyforming in the zone two separate sections of operation for the ester andthe entraining oil respectively, the entraining oil section beingadjacent to said opposite end; withdrawing vapors from said opposite endand condensing them; separating from the condensed vapors thenon-aqueous constituent thereof and returning the same to the entrainingoil section of the distilling zone; causing the liquid in the esteroperation section of the distilling zone to decant, thereby forming twolayers, one of which consists largely of acetic ester; withdrawing thelatter layer from the distilling zone substantially at the same rate asthe acetic ester is fed to the distilling zone in the form of acomponent of said solution; boiling said layer to dehydrate it; and

withdrawing anhydrous acetic liquid from the distilling zone at a pointadjacent the heated end thereof.

5. A continuous process for producing anhydrous acetic acid from asolution consisting largely of ethyl acetate and of aqueous acetic acid,which comprises introducing the solution into the middle part of adistilling zone containing ethyl acetate and an oil having a boilingrange of 84-85 0.; heating the distillingzone at an end thereof to setup in the same a temperature gradient from said end to the opposite endthereby forming in the zone two separate sections of operation for theethyl acetate and the said oil respectively, the said oil section beingadjacent to said opposite end, withdrawing vapors from said opposite endand condensing them; separating from the condensed vapors thenon-aqueous constituen thereof and returning the same to the entrainingoil operation section of the distilling zone; withdrawing liquid fromthe ethyl acetate section of the distilling zone, substantially at thesame rate as ethyl acetate is fed to the distilling zone in the form ofa component of the solution; and withdrawing anhydrous acetic acid fromthe distilling zone at a point adjacent the heated end thereof.

HENRI MARTIN GUINOT.

